For myself it depends on the terrain I am hunting. Each situation will require different requirements. I've hunted 8 ft off the ground in one spot with success, and I've hunted in one location in Wisconsin where I needed to tie TWO 20ft bow ropes together just so I could get my bow up into the tree stand. Main thing is to watch scent control, hunt the wind, and limit movement! I've hunted 25-30 feet up, and if you are in the wrong spot at the wrong time, deer can peg you like you have strobe lights attached to your hat! I've experienced the exact opposite hunting 8 ft off the ground... deer walk right under without a hint that I am there... I guess what I am trying to say is that I don't think I need to be 20 ft up everytime I hunt... depends on where I can hide in the tree... if good limbs are in the tree I choose at 12ft, I go 12 feet... if they are 28', I go 28 feet...
Regardless of what height you hunt... just be sure to where a safety restraints!